The invention relates to an electrochemical method of abatement of chemical oxygen demand in foundry residues or other industrial wastes containing non aqueous components such as oily compounds, glycols or waxes, which make the relevant treatment difficult and expensive. Foundry waste-waters require treatments aimed at abating a remarkable amount of organic substances accumulated during the various process steps in order to allow their reuse or disposal.
The typical waste of an aluminium foundry, which represents one of the most complex and common industrial cases, consists of tap or well-water polluted by release agents (such as alkylaryl siloxanes and ethoxylated polyethylene waxes), hydraulic fluid normally consisting of glycol mixtures (for instance ethylene-propylene glycol), emulsifying agents, paraffin mineral oils, synthetic oils (for instance carboxylic ester-based oils), antifoams (for instance silicone-type ones), oxidation inhibitors (such as boric amide), biocides, complexing agents (such as EDTA) besides dust particles and fat residues. The chemical oxygen demand (COD) of a foundry residue is therefore not only very high (with typical values of 40,000 mg/l) but also deriving from a very complex mixture of components. In order to reduce COD to values allowing the reuse of such waste stream as process water (i.e. values not higher than 1,000 and preferably lower than 500 mg/l of oxygen) a combination of treatments of various kinds can be used, with a series of associated drawbacks. As it could be verified, the treatment by thermocompression in suitable concentrators, which has the advantage of not requiring the employment of particularly qualified staff for its operation, is not sufficient to bring COD below the required threshold and is inevitably affected by a drag of components such as hydrocarbons, glycols and surfactants in the condensate, requiring a post-treatment. In addition or as an alternative, it is possible to carry out a combined treatment of deoiling, ultrafiltration and reverse osmosis, which conversely has to be managed by dedicated personnel especially in the extremely delicate stage of rinsing of the relevant membranes. A combination of chemico-physical and biological processes often lends good results but is made difficult by the variability in the concentration of pollutant species to be treated, besides entailing a heavy handling of chemical reactants and the need for specialised staff. All of these problems finally apply to chemical oxidation treatments with Fenton's reagent followed by ultrafiltration and reverse osmosis which nevertheless, if accurately managed, can provide a high quality water.
For some specific applications it is known that electrochemical waste-water treatments, sometimes attractive for the simplicity of the process and for the very competitive costs, may be taken into consideration; most of COD components in a foundry waste could for instance be abated by oxidation on the surface of an anodically-polarised electrode, after a possible addition of salts to the waste for the sake of imparting a sufficient electrical conductivity. On the other hand, the untreated waste contains pollutant species liable to form pitches and oligomers which would lead to the quick fouling of the electrode surface and its consequent deactivation. Wishing to make an attempt at an electrochemical treatment on a foundry waste, a preliminary deoiling step would at least be required, sensibly decreasing the overall process competitiveness.
It has thus been identified the need of providing a method of treatment of aqueous wastes containing oily compounds, glycols or waxes, in particular of wastes coming from foundry processing, overcoming the inconveniences of the prior art.